The present invention relates generally to industrial fabrics and more particularly to papermaking fabrics.
Generally in the process for making paper, incremental amounts of liquid are removed from a slurry of pulp in a succession of steps. In a first forming step, the slurry is deposited on a porous forming fabric which drains much of the liquid by gravity and suction, and leaves a wet web of solids on the fabric surface. In a later pressing step, the wet web is compressed while on a press fabric in order to removed additional liquid. In a still later, drying step, more liquid is removed by evaporation, usually by supporting the web on a dryer fabric so that the web is in contact with large diameter, smooth, heated rolls.
The papermaking process places considerable demands on the fabrics used in each process step. The fabric should be structurally strong, flexible, abrasion resistant, chemical resistant, contamination resistant, and able to withstand high temperatures for extended times.
A major improvement in the technology of papermaking fabric has been the introduction of synthetic polymer monofilaments. A suitable polymer provides a yarn having mechanical and chemical properties which satisfy the requirements of automated fabric manufacturing and the demands of papermaking.
Fluoropolymer-based yarns are useful because of their high contaminant resistance. Ethylene tetrafluoroethylene polymer (ETFE), for example, is available and can be extruded into yarns. However, ETFE has poor mechanical properties and is difficult to draw without breaking. If one is able to draw the yarn at all, the mechanical properties of the yarn are poor. The poor mechanical properties of ETFE are not surprising given its low breaking or tensile strength.
In the present invention, it was discovered that the addition of an aromatic dicarboxylic acid polymer to a fluorocarbon polymer produces a blend with mechanical properties superior to that of the pure fluorocarbon polymer. Furthermore, the improvement in the mechanical properties, as measured by its breaking strength, was surprisingly large.